The present application relates to a radio communication system for performing radio communication by using a plurality of antennas and also to a communication method for the radio communication system.
In recent years, a radio communication function has been mounted not only on information processing equipment such as a personal computer and communication terminal equipment such as a mobile phone and a PDA (Personal Digital Assistant), but also on audio equipment, video equipment, camera equipment, and a printer. In association with this trend that a radio communication function has been mounted on various equipment, an antenna for transmitting or receiving radio waves is required to have various forms and characteristics.
In some case, such equipment having a radio communication function includes a plurality of antennas for producing different polarized waves for transmission or reception in order to increase a communication speed between transmitting radio equipment and receiving radio equipment. In the case of performing communication by using a plurality of different polarized waves, it can be theoretically said that communication can be performed by using the plural antennas respectively corresponding to different polarization planes. However, in actual, the plural antennas are mounted in the radio equipment so that the polarization planes are orthogonal to each other to suppress interference between the polarization planes.
In such equipment, the region occupied by the antennas becomes large. Japanese Patent Laid-open No. 2005-184564 discloses an antenna device having a substrate formed of a solid electrolyte and two antenna patterns formed of conductive plastic. The antenna patterns are provided on both surfaces of the substrate to receive and/or transmit different polarized waves orthogonal to each other.
There will now be described a change in communication sensitivity according to the polarization planes formed by a transmitting antenna and a receiving antenna. For example, as shown in FIG. 14, a transmitting antenna 30 and a receiving antenna 40 are respectively located at points G and H spaced apart from each other in the direction of the Z axis in an orthogonal three-dimensional coordinate system composed of X, Y, and Z axes orthogonal to each other.
In the case that three orthogonal antenna elements respectively extending in the directions of the X, Y, and Z axes are located at each of the points G and H, it is considered that communication between the three antenna elements of the transmitting antenna 30 and the three antenna elements of the receiving antenna 40 can be respectively performed by using three independent polarized waves obtained by respectively opposing the polarization planes formed by the three antenna elements of the receiving antenna 40 to the polarization planes formed by the three antenna elements of the transmitting antenna 30.
However, since the point G of the transmitting antenna 30 and the point H of the receiving antenna 40 lie on the same straight line extending in the direction of the Z axis, the propagation component of radio waves transmitted from the Z-axis antenna element of the transmitting antenna 30 is largely attenuated before reaching the point H, and therefore may not be received by the Z-axis antenna element of the receiving antenna 40. Accordingly, the polarization planes effectively usable for the communication are formed in the radial directions about the X axis and in the radial directions about Y axis, so that the communication between the remaining two antenna elements of the transmitting antenna 30 and the remaining two antenna elements of the receiving antenna 40 can be respectively performed by using two independent polarized waves.
Further, in the case of performing short-distance communication such that a transmitting antenna and a receiving antenna are spaced apart from each other by a short distance several times the wavelength of radio waves for use in the communication, independent polarized waves as mentioned above are used. Also in the case of performing long-distance communication such that a transmitting antenna and a receiving antenna are spaced apart from each other by a long distance sufficiently larger than the above wavelength, independent polarized waves as mentioned above are used.
Such short-distance communication is applied to a noncontact type IC card, for example, without the use of a connector or the like for electrical connection, and a high communication speed is desired with the feature missing in the long-distance communication.